Dispersions of carbon nanotubes by helical flavin surfactants: Solvent induced stability and chirality enrichment, and solvatochromism

Although solvent governed stability, selection and optical behavior of single walled carbon nanotubes (SWNTs) brought about by using specialized surfactants have importance in relation to various optoelectronic and biological applications, their origins and solvatochromism have not been thoroughly explored owing to the lack of ideal surfactant dispersing SWNTs. We show that solvent driven SWNT selection and solvatochromism occurs in SWNTs wrapped by two flavin derivatives in solvents with dielectric constants (ε) ranging from 2.3 to 80.4. Flavin-SWNT nanocomposite dispersions exhibit near-armchair and chirality-specific enrichment in lower ε aromatic solvents, and those with broader chirality selectivity are formed in higher ε solvents. Those trends are also observed by controlling flavin concentration. Consideration of optical spectra provides relative quantum yields of flavin-SWNT in each solvent, showing toluene and p-xylene dispersions as best owing to highly individualized SWNT. The selectivities and relative stabilities of the flavin-SWNT nanoconjugates are associated with the solubility parameter (δ), which controls the respective morphology of the side chains and the relative Gibbs energies of the nanoconjugates relative to those of the free flavin and SWNTs. Differences between δ of side chains vs. the media results in extended and collapsed side chain conformations in respective lower and higher ε solvents, leading to diameters (d_t) selection of SWNT owing to diameter modulation of the isoalloxazine assembly. In addition, with increasing ε optical transitions of flavin/SWNT exhibit lower bathochromic shifts owing to a passivating role played by the isoalloxazine that does not take place with other surfactants. The results suggest new guidelines for the design of good SWNT dispersions that take into consideration self-association of the surfactant, conformations of side chains and the nature of the solvent.